Let's say one adult could educate 30 kids. That's "efficient" compared to one adult educating one or two or three kids. Okay?
But it doesn't mean everything in all cases. If one parent is going to be home all the time, then fine. If he or she is capable of being as effective a teacher, fine again.
The such a measure of efficiency wouldn't be useful, and the efficiency you talk about (hours the child spends learning, etc) is a more appropriate indicator.
Hey, I wasn't kicking homeschooling. It is right for some people. I just don't think it is right for everyone. Not every parent is a "bad person" if they have interests outside of doing their own children's schooling. But for those who want to, fine.